Given a string s, find the length of the longest substring without repeating characters.

To find the length of the longest substring without repeating characters, we can use the sliding window approach. Here's the algorithm:

1. Initialize a set to store the unique characters in the current window, and two pointers left and right to mark the current window.
2. Initialize a variable max_len to 0 to store the length of the longest substring without repeating characters.
3. While the right pointer is less than the length of the string s:
- If the character at s[right] is not in the set, add it to the set and move the right pointer to the right.
- Else, remove the character at s[left] from the set and move the left pointer to the right.
4. Update the value of max_len with the maximum of its current value and the size of the set.
5. Return max_len.

Here's the Python code for the algorithm:

def longest_substring(s):
    unique_chars = set()
    left = 0
    max_len = 0
    
    for right in range(len(s)):
        while s[right] in unique_chars:
            unique_chars.remove(s[left])
            left += 1
        unique_chars.add(s[right])
        max_len = max(max_len, len(unique_chars))
    
    return max_len

Example usage:

s = "abcabcbb"
print(longest_substring(s)) # Output: 3 (for "abc")